Global Positioning System

　　Global Positioning System (GPS) is a mid-range circular orbit satellite navigation system. It can provide accurate positioning, velocity measurement and high precision time standards for the vast majority of the Earth's surface (98%). Developed and maintained by the U.S. Department of Defense, the system provides continuous and accurate 3D position, motion, and time determination for military users located anywhere in the world or in near-earth space. The system includes 24 GPS satellites in space; One main control station, three data injection stations and five monitoring stations on the ground as well as the GPS receiver on the client side. With at least three of these satellites, the user's position and altitude on the earth can be quickly determined. The more satellites we can connect to, the more accurate the decoded position will be.

　　The system was developed by the U.S. government beginning in the 1970s and completed in 1994. Users only need to have a GPS receiver and do not have to pay extra. GPS signals are classified into two types: SPS (Standard Positioning Service) and PPS (Precise Positioning Service). Because SPS without any authorization can be used freely, the original United States for fear of hostile countries or organizations will use SPS attack on the United States, so in the civil signal artificially added error to reduce its accuracy, so that its final positioning accuracy is about 100 meters; The accuracy of military regulations is under ten meters. After 2000, the Clinton administration decided to eliminate interference with civilian signals. Therefore, now civilian GPS can also reach about 10 meters of positioning accuracy.

　　GPS system has the following advantages: all-weather, not affected by any weather; Global coverage (up to 98%); High precision of three-dimensional constant speed and timing; Fast, time-saving, high efficiency; Widely used, multi-functional; Mobile positioning; Different from the dual-star positioning system, the receiver does not need to send out any signal during use, which increases the concealment and improves the effectiveness of its military application.

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